Field of Invention
This invention relates to plain bearing-like devices and more particularly to precision, ultrastiff plain bearing-like structures both macroscopic and microscopic with frictional and structural properties that can be electronically controlled and which contain a built-in ultrastiff force sensing mechanism.
Precision bearings, specifically designed to translate or rotate a mechanical load with submicron accuracy have become increasingly important in the optics, semiconductor manufacturing, and micromachining industries. Unlike regular bearings that are used, for example in automobiles, these precision bearings have unique primary and secondary characteristics. Among the most important primary characteristics are (1) extremely high stiffness, (2) controllable frictional characteristics, (3) enhanced smoothness-of-travel, and (4) superior accuracy-of-travel. Some of the secondary features are (a) high damage threshold, (b) high reliability, (c) low maintenance, (d) low external support requirements, and of course (e) low cost.
A common bearing, found in many semi-precision, mechanisms, is the sliding bearing or plain bearing which coincidentally also exhibits many of the primary and secondary characteristics of a precision bearing. As used herein, the term “plain bearing” simply means that the bearing's load is supported through sliding motion between two solid surfaces. FIG. 1A illustrates a lubricated planar configuration of a simple plain bearing, which consists of a flat bar-shaped upper slide-plate 50 and a similar lower slide-plate 54 sliding against each other with a lubricant 52, such as oil, between the sliding surfaces. FIG. 1B shows two forces acting on the upper slide-plate of the simple plain bearing of FIG. 1A. The force, FAPP 45, is an applied force sufficient to maintain contact along an interface between the surfaces of the two slide-plates. The other force is an applied sliding force 46, which, along with the component of the force 45 parallel to the interface, can be used to translate the two slide-plates relative to each other.
A plain bearing is characterized by (1) its inherent simplicity, having a minimum number of moving parts; (2) its superior runout or off-axis error characteristic which is obtained by averaging any local smoothness imperfections over the entire sliding surface area; (3) its high shock-loading or damage threshold which is the result of spreading the shock force on the bearing over a large surface area, and (4) its extremely high compressive stiffness since the bearing has large direct material to material sliding contacts.
However, despite all of these advantages, a lubricated plain bearing is not generally used in precision applications for several reasons. The first reason is because of the bearing's relatively high frictional forces generated by the component [FAPP]Z of the force 45 which is perpendicular to the interface. These frictional forces are a direct result of its high coefficient of friction, which can be ten to one hundred times larger than an equivalent ball or air bearing. The second reason is because of stiction, an extra frictional holding phenomenon, above the “normal” static friction, that occurs when two extremely smooth and lubricated contacting surfaces that were stationary, start to slide. The third reason is because of a phenomenon known as stick-slip which results in fluctuations in the frictional forces while the bearing is in motion. The cumulative effects from all three phenomena associated with a plain bearing, will usually render an instrument equipped with this type of bearing unable to perform precise and microscopic movements.
In prior art, there are three general methods used to change the frictional forces between two sliding surfaces. First, the actual coefficient of friction between the sliding surfaces can be modified by very thin films or coatings with good tribological properties. This is generally accomplished by using some combination of a solid, liquid or gas as reviewed in U.S. Pat. No. 4,944,606 of Lindsey et al., (1990) and U.S. Pat. No. 5,911,514 of Davies et al., (1999). Second, if one or more operating parameters between the two sliding surfaces can be altered, the frictional characteristics can be changed. Some common operating parameters that can be readily controlled to produce a relatively small variation in frictional behavior are surface temperature, as described in U.S. Pat. No. 5,441,305 of Tabar (1995) and sliding speed, as shown in U.S. Pat. No. 5,043,621 issued to Culp (1991). Finally, if the compressive force between the two sliding surfaces is minimized, or time-modulated as revealed in U.S. Pat. No. 3,774,923 of Suroff (1973), U.S. Pat. No. 3,756,105 of Balamuth et al., (1973), and U.S. Pat. No. 4,334,602 of Armour et al., (1982), or even totally eliminated by, for example, magnetic levitation as shown in U.S. Pat. No. 3,937,148 issued to Simpson (1976), then the frictional force generated between these two surfaces is correspondingly minimized.
All these techniques of friction reduction can be used individually or possibly, in some cases, in combination. Examples are air bearings, like those described in U.S. Pat. No. 3,683,476 of Lea et al., (1972), used in precision translational stages where a combination of an air film and levitation techniques are employed to reduce friction.
Solutions to the stiction phenomenon were revealed in applications related to magnetic disk storage. U.S. Pat. No. 4,530,021 issued to Cameron (1985) and No. 6,002,549 issued to Berman et al., (1999), teach that vibrational techniques, which dither the slider head also free it from the force of stiction. Further solutions to the stiction problem involve surface texturing techniques, as illustrated by U.S. Pat. No. 5,079,657 of Aronoff et al., (1991), which allow a slider head to leave a surface smoothly.
Similarly, the stick-slip problem has also been successfully addressed in diverse fields, such as those including focusing mechanisms used in optical disk storage and wiping mechanisms for automotive windshields. Their solutions, like the stiction case, are also based upon oscillation methods as revealed in U.S. Pat. No. 4,866,690 issued to Tamura et al., (1989) and U.S. Pat. No. 5,070,571 issued to Masuru (1991).
However, all these prior art techniques suffer from one or more drawbacks which render their benefits, in a plain bearing-like device application, individually insufficient and in some cases, incompatible with precision. For example, simply adding a lubricant between the two surfaces of a plain bearing does not minimize either stiction or stick-slip behavior which are required for smooth microscopic motion as well as positioning accuracy. Controlling the aforementioned operating parameters does not reduce the coefficient of friction enough for most applications. Changing the compressive force by periodic or continuous levitation compromises bearing compressive stiffness, produces changes in the bearing position depending upon whether the bearing is levitated or not, and may require substantial external facility support such as, in the case of air bearings, a continuous supply of clean, dry air delivered at a regulated pressure. Furthermore, U.S. Pat. No. 4,648,725 of Takahashi (1987), teaches that positioning devices employing bearings with only a fixed, very low coefficient of friction, such as an air bearing, tend to have an extended settling time even after moving at relatively low velocity to a particular position of interest.
Also, prior art examples using vibrational or oscillation techniques to minimize stiction or stick-slip make no effort to separate the vibrational motion from the ideally desired motion of the load. Separation of the two motions is paramount in precision bearings where maintaining the integrity of the slidable path, continuously contacting surfaces, and vibrational insensitivity are all desired properties. Furthermore, prior techniques that use the vibrational motion only prior to moving the load along its desired path do not remove any stick-slip problem from occurring during travel.
Therefore, in order for any plain bearing-like devices to be successfully employed for precision and microscopic movement applications, simultaneous solutions to the high friction, short settling time, stiction, and stick-slip issues must be implemented utilizing techniques obtained from many different technical areas.
Furthermore, in order to utilize such a plain bearing-like device as “real” ultrastiff bearing, an important additional bearing characteristic must be addressed. In prior art, when bearings are incorporated into a stage for example, there is generally an incompatibility between the ultrastiff requirement and the bearing's mechanical tolerances. Ideally, for a stage to be truly ultrastiff in one or more axes orthogonal to the direction of travel, all the components of the stage must have essentially zero mechanical tolerance or “play” in these orthogonal directions. In prior art, the solutions to this problem are (1) to simply minimize these mechanical tolerances by manufacturing mechanical components to nearly exact specifications, or (2) to compensate for the mechanical variations by incorporating elastic members between moving surfaces, or (3) to maintain an equivalent “zero tolerance” condition by dynamically adjusting one or more mechanical components. Of these heretofore mentioned solutions, only the actively servoed technique of dynamic compensation approaches the ideal mechanical stiffness requirements and this dynamic adjustment technique has been successfully implemented in the form of an air bearings, like those described in U.S. Pat. No. 3,683,476 of Lea et al., (1972). Therefore, for success in ultrastiff applications, a plain bearing-like device must also contain a solution to this incompatibility between mechanical tolerance and stiffness.
To complete the prior art survey, a seemingly unrelated device known as the ultrasonic bonder should also be included.
The ultrasonic bonder or wire bonder is commonly used in the semiconductor industry to electrically connect the integrated circuit chip to the leadframe pins. Ultrasonic bonding, also known as welding or friction-fusion bonding, is a process for joining two materials by means of a bonding tool which exerts a normally directed clamping, mashing, or applied force on two juxtaposed contacting surface areas while vibrating one parallel to the other at an ultrasonic frequency. As a result, local plastic deformation takes place in the interfacing materials and a metallurgical bond is formed between the two materials. The original thermosonic bonder, as described in U.S. Pat. No. 3,054,309 of Elmore (1962), and its many descendants, typified by U.S. Pat. No. 5,186,378 issued to Alfaro (1993), generally requires not only a clamping or mashing pressure (around 200 MPa) and ultrasonic vibrations (around 60 kHz) but also high temperatures (around 175° C. to 300° C.) to obtain near 100 percent intermetallic coverage. Recent advances include an ambient temperature (around 27° C.) ultrasonic wire bonder, as revealed in U.S. Pat. No. 5,244,140 of Ramsey et al., (1993), and a thermoplastic welding apparatus described in U.S. Pat. No. 4,482,421 issued to Gurak (1984).